In a large bowl, use an electric mixer to combine the butter and peanut butter. Beat until smooth and well-combined. Mix in 1/4 cup milk, then add in the sugar a little at a time until it is all mixed in. Sprinkle in a little more milk, if needed, and continue to beat until the frosting is a good spreading consistency. It should be creamy and delicious!
You can use this recipe to frost either a two-layer cake or 24 cupcakes.